This invention relates generally to electromagnetic actuators and, in particular, an improved compact electromagnetic actuator usable for driving a blade mechanism in photographic apparatus.
A wide variety of actuators have been used for driving shutter mechanisms in cameras. Typically, these tend to be mechanically sophisticated in construction and operation. Many known actuators include a stored energy device in the form of a mechanically tensioned spring. While they function satisfactorily and provide commercially acceptable reliability, nonetheless, they have certain limitations. For instance, they typically have an intricate construction and are relatively costly and difficult to manufacture. Moreover, the performance characteristics of spring-biased mechanisms are relatively fixed. This is disadvantageous since their performance characteristics cannot be programmable. Furthermore, spring biased shutters have dynamic performance characteristics which are not as accurate or as reliable as they might otherwise be.
Recent efforts have aimed at improving upon such shutter mechanisms. This is not only because of the limitations mentioned above, but also due to the desire to miniaturize cameras.
Some proposals have suggested using electromagnetic actuators for driving the shutter blades. Electromagnetic actuators employing annular field coils with magnetic armatures are considered too bulky and, therefore, less than ideal for compact shutter applications.
Other known kinds of electromagnetic actuators use conductor coils in a shutter blade itself for purposes of lessening its moment of inertia, improving its dynamic characteristics, and enhancing actuator miniaturizaton. U.S. Pat. Nos. 4,024,552, 4,348,092 and 4,348,094 are illustrative of these latter kinds.
Despite their relatively compact arrangement, however, such actuators are not as powerful or as efficient as is desirable. This is disadvantageous for exposure control purposes, since it is often necessary to produce relatively large amounts of driving power. In this regard, known coil arrangements in such actuators have a relatively limited effective coil length in the magnetic fields. This, thus, limits the generated driving power. In this regard, the driving power produced by the conductor coils is proportional to the length L of its components within the flux, the magnetic flux density of the magnetic field B, and the driving current I. Thus, if the length of the coil is limited, the driving power is correspondingly limited.
At the same time, however, it is highly desirable to have an actuator as efficient as possible. Hence, it will be appreciated then that it is commercially desirable to have actuators produce relatively high driving forces with use of as little power as needed, yet be as compact as possible. Further, it is desirable to improve efficiency, but in a way which does not introduce other problems that could hamper dynamic performance and the like. For instance, it would be undesirable to have forces generated in such a system cause the armature to deviate from its intended path.